With little fanfare, and on a tight budget, the United States is reshaping its space program in an attempt to return men to the moon by 2020, and eventually land humans on Mars. It is a journey that poses formidable scientific, political and financial challenges.

In May 1961, a speech by U.S. President John F. Kennedy changed the course of human history.

"The goal, before this decade is out, is of landing a man on the moon and returning him safely to the Earth," said the president.

In July 1969, Neil Armstrong became that man -- the first human to walk on a celestial body other than Earth.

Thirty-five years later, in January 2004, President George W. Bush called for a new vision of space exploration: "We will build new ships to carry man forward into the universe, to gain a new foothold on the moon, and prepare for a new journey to the worlds beyond our own."

Though overshadowed by the war in Iraq, combating terrorism and other issues, Bush's vision has quietly become the driving force in remaking America's space program.

Returning men to the moon is central to the vision, says Dr. Paul Spudis, a lunar scientist at Johns Hopkins University, who served on the president's space exploration policy commission.

"As we get more and more experience on the moon -- more and more experience working with the moon itself -- we will start to make things we can use on the moon from the moon,” said Spudis. “And once you've done that you've taken the first step toward cutting the cord with the Earth."

Five steps need to be taken to realize the president's vision. First is fixing the Space Shuttle and getting it back in flight. Second is completing the International Space Station, using the Shuttle as a workhorse to move critical parts and supplies.

"The third milestone is to develop a new human space vehicle,” according to Spudis. “They call it the Crew Exploration Vehicle, CEV. That will allow us to go not only into Earth orbit, but beyond Earth orbit, into missions to the moon, and ultimately to Mars and other places.

The fourth piece of the vision is to return to the moon -- to return to the moon to do science, to learn how to explore, and to learn how to extract what we need from the resources of the moon. And the last milestone of the vision is the Mars mission -- a mission to Mars and to other places."

Realizing this vision is the job of the National Aeronautics and Space Administration -- NASA -- headed by Dr. Michael Griffin. "Completing the International Space Station, retiring the Shuttle by 2010; managing the effective transition from the Shuttle to the new crew exploration and crew launch vehicles are fully the equal of the tasks which were set before any earlier generation.... I believe we can carry this out,” said NASA’s administrator.

But Congressman Mark Udall, a Democrat from Colorado, is among those who are not so sure. "I think many of us in the Congress, both on the Democratic side and the Republican side, think we are asking NASA to do too much with too little when it comes to resources."

Dr. Griffin acknowledges that the costs of manned space exploration require cutbacks in other NASA programs such as science and aeronautics. "We cannot afford to do everything that each of our constituencies would like us to do."

But Congressman Bart Gordon, a Democrat from Tennessee, is doubtful. "The reality is that the glowing vision Congress was given two years ago bears little resemblance to the situation at hand. It's become painfully obvious to me that we're not going to get there from here."

Dr. Spudis of Johns Hopkins University is more optimistic. "They're getting over $16 billion a year. And that's a lot of money. And I think that depending on how they approach the problem, and how they begin to do it, I think they can go back to the moon on that amount of money. I think its going to require some clever innovation; it's going to require some imaginative management; but its certainly doable. And if its not doable then there is no future for the space program because they're not going to get much more than that."

If the Bush vision does return us to the moon by 2020, it will be 50 years after the Kennedy vision brought that first giant leap for mankind.